1. Field of the Invention
This invention relates to capacitors, and more particularly, methods of controlling electrical quantities associated with them.
2. Description of the Related Art
Capacitors are often used for decoupling and bypassing of noise for power distribution systems, and have a multitude of other uses in electronic circuits. Selection of capacitors for a particular application is based in large part on their various electrical quantities. FIG. 1 shows a simple schematic of a capacitor equivalent circuit for one embodiment of a capacitor. For a given capacitor, the electrical quantities (i.e. characteristics) include a capacitance C, an equivalent series inductance ESL, and an equivalent series resistance ESR. The ESL and ESR components represent parasitic electrical quantities. All three of these electrical quantities may be frequency dependent. For example, ESR occurs at a frequency (i.e. the resonant frequency) where the capacitive and inductive reactances of the capacitor cancel each other out. The inductive reactance of the capacitor may increase for frequencies above the resonant frequency. Broadly speaking for a given capacitor, at frequencies below the resonant frequency, the impedance is generally capacitive, while above the resonant frequency the impedance is generally inductive. At resonant frequency, the impedance of a capacitor is resistive. FIG. 2 illustrates an impedance profile for one embodiment of a typical capacitor.
One type of capacitor that is commonly used in electronic systems is a multi-layer ceramic capacitor (MLCC). FIG. 3 is a side view of one embodiment of a multi-layer ceramic capacitor. In part because of the number of capacitor plates, a multi-layer ceramic capacitor may exhibit a relatively large capacitance in a small package. The small body size may help to limit the amount of inductance. The resistive losses in the conductors depend on various factors such as the number and size of metal plates, the thickness and conductivity of the metal plates, and the resistance of the capacitor terminals.
For some applications, a low value of ESR is desirable. However, other applications exist where a moderate or even high ESR is desirable. In general, the optimum value for a capacitor""s ESR is dependent upon the application in which it is implemented. Various methods exist for increasing the ESR for a given capacitor. One method for increasing ESR is to place a resistance (e.g., from a discrete resistor) in series with the capacitor. While this method may increase the ESR, it may also add inductance from the discrete resistor. Alternatively, Tantalum and niobium capacitors, which may have higher ESR values for the same capacitance, may be used instead of multi-layer ceramic capacitors. However, tantalum and niobium capacitors may also be significantly larger than multi-layer ceramic capacitors and may also have a greater inductance. Thus, these solutions may not satisfy the requirements of a higher ESR value without adversely affecting the inductance.
A method for constructing a capacitor having an increased equivalent series resistance (ESR) is disclosed. In one embodiment, a capacitor includes a plurality of capacitor plates (at least one pair of plates, with each plate of the pair coupled to a different terminal) comprised of a conductive material and first and second capacitor terminals. At least one of the capacitor plates is coupled to the first terminal and at least one of the capacitor plates is coupled to the second terminal. At least one of the plurality of capacitor plates includes a pattern, wherein the pattern is void of conductive material. The void in the conductive material formed by the pattern may cause a path of current flow through the capacitor plate to be substantially altered in comparison to a capacitor plate that is continuous. By using capacitor plates having voids of conductive material that cause the current path to be altered in comparison to continuous capacitor plates, a capacitor can be constructed having a higher ESR.
In one embodiment, a plurality capacitor plates is coupled to the first capacitor terminal and a plurality of capacitor plates is coupled to the second capacitor terminal. An entire edge of each capacitor plate may be connected to its respective terminal. Alternatively, an edge connection including only a portion of the edge (wherein portion is not co-dimensional with the edge) may be used to connect a capacitor plate to its respective terminal.
One or more of the capacitor plates may include the patterns that are void of conductive material. The capacitor may be divided into zones, with the patterns confined to one of the zones. This may ensure that the equivalent series inductance (ESL) of the capacitor is substantially the same as a comparable capacitor wherein none of the plates include patterns that are void of conductive material. Similarly, the patterns void of conductive material may be sized and shaped such that the capacitance value of a capacitor with these plates is substantially the same as a comparable capacitor wherein the capacitor plates are continuous.
In some embodiments, a pattern that is void of conductive material may be created by the manner in which one or more of the capacitor plates are connected to their respective terminals. In such embodiments, only a portion of the edge of a capacitor plate is electrically coupled to its respective terminal. Thus, a void of conductive material exists between the capacitor plate and the respective terminal, and all current flow between the capacitor plate and the terminal is forced to pass through the reduce-size area where the electrical connection is made.